1. Field of the Invention
This invention relates to hydraulic rotating equipment suited for use as a hydraulic pump, hydraulic motor or the like, and also to a working machine provided with the hydraulic rotating equipment.
2. Description of the Related Art
In general, hydraulic rotating equipment which are widely used as hydraulic pumps, hydraulic motors and the like are each provided, for example, with a cylindrical casing forming an outer shell, a rotating shaft connected to an output shaft of a prime mover and rotatably arranged in the casing, a cylinder block defining therein a plurality of cylinders formed at intervals in a circumferential direction of the rotating shaft, and a like plurality of pistons accommodated in the plurality of cylinders, respectively, of the cylinder block and reciprocable in association with rotation of the cylinder block.
Such hydraulic rotating equipment is also provided with shoes, a swash plate, and a valve plate. The shoes are held slidably with end portions of these plural pistons, are rotatable together with the cylinder block, and are in slide contact with the swash plate. The valve plate is in slide contact with an end surface (rear end surface) of the cylinder block, said end surface being on a side opposite to the swash plate, and defines therethrough a low-pressure port and a high-pressure port intermittently communicable with the cylinder block under rotation. On a surface of the valve plate, said surface being maintained in slide contact with the cylinder block, there is formed a seal land that seals hydraulic oil from the low-pressure port or high-pressure port. By this seal land, it is possible to suppress the leakage of hydraulic oil from the low-pressure port or high-pressure port.
When the hydraulic rotating equipment configured as described above is used as a hydraulic pump, upon rotation of the rotating shaft by an output from the prime mover, the cylinder block rotates together with the rotating shaft so that each piston reciprocates. At this time, the hydraulic oil flows into each cylinder of the cylinder block from the low-pressure port of the valve plate, and by the corresponding piston, is pressurized and delivered from the high-pressure port of the valve plate.
When the hydraulic rotating equipment is used as a hydraulic motor, on the other hand, the flowing of high-pressure hydraulic oil from the high-pressure port into each cylinder of the cylinder block allows the hydraulic oil, which has flowed in, to act on the corresponding piston. At this time, the piston is pressed against the side of the swash plate under the hydraulic pressure of the hydraulic oil. After rotating the rotating shaft together with the cylinder block, the hydraulic oil is, therefore, returned to a hydraulic oil tank from the low-pressure port.
When the hydraulic rotating equipment is used as the hydraulic pump, the cylinder block generally rotates in one direction. When the hydraulic rotating equipment is used as the hydraulic motor, on the other hand, the hydraulic rotating equipment is designed such that the cylinder block can rotate in two directions, in other words, can undergo both forward rotation and reverse rotation. By reverse rotation of the cylinder block, the high-pressure port and low-pressure port of the valve plate, therefore, change with each other.
The slide contact surface of the valve plate as a stationary element and that of the cylinder block as a rotating element are designed such that balance can be maintained between force, under which the cylinder block is pressed against the valve plate by hydraulic pressure, and static pressure, which is caused by leakage of hydraulic oil to the slide contact surfaces of the valve plate and cylinder block, in order to suppress a reduction in volumetric efficiency as a result of leakage of the high-pressure hydraulic oil. In particular, the hydraulic oil leaks in a large amount from the high-pressure port. Accordingly, the slide contact surfaces of the valve plate and cylinder block have been often designed to make smaller the clearance between the seal land of the valve plate, said seal land being on the side of the high-pressure port, and the cylinder block, and seizure has tended to occur at the seal land around the high-pressure port.
As one of related art that can prevent seizure of slide contact surfaces of a valve plate and a cylinder block, an axial plunger hydraulic pump or motor has been proposed (see, for example, JP51-B-14282). The axial plunger hydraulic pump or motor is configured as will be described hereinafter. A seal land extends along substantially a half part of a high-pressure port of a valve plate, said half part being on a side where a port, to which cylinder ports are to be connected, changes from a low-pressure port to the high-pressure port during operation of the pump or motor, and is located on a side inner than pads arranged on an outer circumference of the valve plate. In a seal surface of an outer portion of the seal land, said seal surface facing an end surface of the cylinder block, bottomed concavities are arranged. Hydraulic oil leaked from the high-pressure port is allowed to fill the bottomed concavities such that the effective component of press-back force, which is produced by the hydraulic oil between the seal surface of the outer portion of the seal land and the end surface of the cylinder block, can be increased.
In the above-mentioned axial plunger pump or motor of the related art, a contrivance has been made to broaden the clearance between the seal land of the valve plate on the side of the high-pressure port and the seal land by increasing the effective component of the press-back force with the hydraulic oil in the bottomed concavities arranged in the outer part of the seal land of the valve plate. However, pads are arranged over the entire periphery of an outer side of a seal land on the valve plate, and therefore, the slide contact area between the valve plate and the cylinder block increases as much as the pads. Therefore, the friction force that the cylinder block receives from the slide contact surface of the valve plate during rotation increases, leading to a concern that a torque loss may increase in association with rotation of the cylinder block.
With such actual circumstances of the related art in view, the present invention has as objects thereof the provision of hydraulic rotating equipment capable of reducing a torque loss that occurs in association with rotation of a cylinder block and also a working machine provided with the hydraulic rotating equipment.